1. Field of the Invention
The present invention relates to a method for fabricating semiconductor devices, and more particularly to a method to prevent the formation of a thinner portion of gate oxide at the junction between the side walls and the bottom insulator of a trench type semiconductor device.
2. Description of the Prior Art
As the design rule continues to shrink, a trench type semiconductor device, for example metal-oxide-semiconductor transistor, has been provided to economize the wafer area.
Referring to FIGS. 1A through 1C, the cross-sectional side views of a conventional method for fabricating a gate oxide within a trench are depicted in sequence.
Referring now to FIG. 1A, a cross-sectional view of the first step is schematically shown. FIG. 1A shows a silicon substrate 10 having a trench 12. A silicon nitride layer 11 is formed overlaying the silicon substrate 10.
Next, as shown in FIG. 1B, a bottom oxide 16 is deposited on the lower portion of the trench 12 by high density plasma chemical vapor deposition (HPCVD).
Referring now to FIG. 1C, a gate oxide layer 18, for example silicon oxide having a thickness of about 500 angstroms, is grown on the side walls of the trench 12.
However, the gate oxide layer 18 is naturally thinner at the junction 20 between the side walls and the bottom oxide 16. This thinner portion will cause junction leakage problems.